Identification of Novel Oxazolo[5,4‐ <i>d</i> ]pyrimidines as IL‐33 Inhibitors for Immuno‐Oncology Therapy

The IL-33/ST2 signaling axis plays a crucial role in shaping the tumor microenvironment, primarily by promoting immune suppression through tumor-infiltrating regulatory T (T_reg) cells. Elevated IL-33 levels are associated with poor prognosis in multiple cancers, as IL-33 enhances T_reg stability, proliferation, and suppressive function, thereby dampening antitumor immunity. In this study, we present the rational design and biological evaluation of novel small-molecule inhibitors targeting IL-33/ST2 signaling to suppress T_reg-mediated immune evasion. Guided by structure-based drug design, we identified oxazolo[5,4-d]pyrimidine derivatives capable of forming key hydrogen bonds within the IL-33 binding pocket, thereby enhancing binding affinity. In vitro and ex vivo assays using T-Cell Receptor(TCR)-stimulated human T_reg cells demonstrated that these compounds significantly suppressed IL-33-induced T_reg activation and proliferation. Notably, KYH1942 showed the most potent activity, reducing IL-33-induced Foxp3 and Ki-67 expression in a dose-dependent manner, indicative of impaired T_reg function. It is worth noting that the inhibitory activities of our compounds are comparable to that of a neutralizing ST2 antibody. To our knowledge, this is the first report demonstrating that small-molecule IL-33 blockade can directly suppress T_regs, including IL-33-driven Foxp3 and Ki-67 expression. These findings establish a novel immunotherapeutic strategy targeting IL-33-mediated immune suppression.